Technical Field
The technical field relates to titanium alloys, components formed therefrom and methods of using such components.
Background Information
Increasing worldwide demand for energy continues to drive extraction/recovery of energy sources to more challenging frontiers, often involving engineering material limitations. This is exemplified in the extraction of geothermal energy and hydrocarbons (i.e., oil/gas), whereby it is necessary to pursue ever deeper fields and wells, on land and in deeper offshore waters, encountering correspondingly higher temperatures and pressures, and more aggressive, corrosive environments. Hydrocarbon reservoirs/wells have been classified as high-pressure/high-temperature (HPHT) when bottomhole temperatures exceed approximately 300° F. and 10,000 pounds per square inch (psi) pressure. Extreme HPHT (XHPHT) wells are those exceeding about 400° F. and 20,000 psi bottomhole pressure. These hot, and often deep, well reservoirs typically produce a mixture of hydrocarbons and aqueous well fluids, including chloride-containing brines pressurized with acidic gases such as carbon dioxide (CO2) and/or hydrogen sulfide (H2S). Wells are now being drilled to total depths of 50,000 feet and beyond where temperature and/or pressure increasingly elevate. Geothermal wells used for energy extraction and power generation are generally shallower with correspondingly lower bottomhole pressures, but can produce very high temperature (e.g., as high as 625° F.) sweet or sour highly-saline brines which are highly corrosive to conventional metallic materials.
Higher strength and fully corrosion resistant alloys for various well components, such as the production tubing string and casing, wellhead valves, bottom well liner, and well logging housing and fluid sampling vessels are required to successfully handle these often sour (H2S-containing) HPHT/XHPHT well fluids. In addition to these downhole well components, offshore hydrocarbon production must consider appropriate production riser tubular strings and components to convey these aggressive HPHT well fluids from the seafloor to the offshore platform. In addition to elevated corrosion resistance, the trend toward field development in deeper and ultra-deep (>5,000 ft. depth) waters also requires higher strength and lighter weight tubular strings for production, export, and re-injection offshore risers, as well as well-workover and/or landing strings. Traditional engineering corrosion resistant alloys or CRAs (e.g., stainless steels and nickel-base alloys) have limited utilization in these situations due to their relatively lower strengths and higher densities (i.e., lower strength-to-density ratios). Even higher strength steel—e.g., high-strength low-alloy (HSLA) steel with up to 150-160 ksi (kilopounds per square inch) minimum yield strength—tubular strings can become too heavy to hang in ultra-deep offshore waters in certain scenarios or in deep oil and gas wells.
In recent years, several higher strength titanium alloys have found successful application in these energy industry arenas over the past 15 years due to various desirable characteristics such as high strength and low densities resulting in elevated strength-to-density ratios (i.e., lightweight structures), elevated corrosion resistance to aqueous chloride fluids (seawater, well fluid brines) and H2S and CO2 acid gases, lower elastic modulus (high flexibility), and excellent air and saltwater fatigue resistance (desirable for dynamic offshore riser components). These include use of Ti-38644 (ASTM Grade 19) beta-titanium alloy in various downhole tubular strings and well jewelry in hydrocarbon and geothermal wells, Ti-64 ELI (ASTM Grade 23 Ti) in an offshore drilling riser, and Ti-64-Ru (ASTM Grade 29 Ti) as titanium stress joints in catenary and top-tensioned steel offshore riser top and bottom terminations and as hypersaline-brine geothermal well production casings in the Salton Sea. More recently, the Ti-6246 alloy has been tested and qualified for oil country tubular goods (OCTG) production tubulars for high temperature sour well service by Chevron.
Traditional, commercial titanium alloys are either: 1) relatively low strength (25-100 ksi yield strength or YS) which are generally used for chemical, power generation, and industrial processes; or 2) higher strength (110-180 ksi YS) alloys designed primarily for high strength-to-weight ratios to achieve lightweight, structurally-efficient aerospace airframes and engine components. Unfortunately, with limited past need for enhanced resistance to halide-containing chemicals, seawater, and various cold or hot brines, these traditional higher-strength aerospace titanium alloys were not designed or intended to resist localized corrosion attack or stress corrosion cracking (SCC) in aqueous chloride media, particularly at higher temperatures and/or lower pH environments. As such, most of these alloys exhibit unacceptably low saltwater fracture toughness (KSCC) values in saltwater and other aqueous chloride fluids, failing to meet fracture mechanics requirement for highly stressed components.
Table 1 in part provides an overview comparison of positive features vs. limitations of higher-strength (≥110 ksi YS) commercial titanium alloys considered and/or used for these energy extraction applications. It can be seen that although the three alloys approved under the ANSI/NACE MR0175/ISO 15156 Standard for sour service (Ti-64-Ru, Ti-6246, Ti-38644) offer varying degrees of hot aqueous chloride/brine resistance, they exhibit other crucial limitations in strength (Ti-64-Ru) especially as temperature increases, or in fusion weldability (Ti-6246 and Ti-38644). Ti-6246 alloy components exhibit relatively low fracture toughness values (precluding their use in offshore risers, or well-workover and landing strings), which are further diminished in aqueous chloride media. The other four alloys are highly susceptible to localized attack and SCC in halide (e.g., chloride-containing) brines, particularly as temperatures increase, and/or are limited in their weldability. The need for fusion-weldability (e.g., gas tungsten arc or GTA welding, gas metal arc or GMA welding, and plasma welding) is primarily a requirement for fabrication of offshore risers and possibly drilling components, and is not relevant for downhole well/OCTG components where seamless products are generally used.
Improving the corrosion resistance of various commercial high-strength alpha-beta and beta titanium alloys through minor PGM (platinum group metal) alloy additions (i.e., Pd or Ru) for hot sour, chloride-rich oil/gas well service has been investigated and documented, for instance in U.S. Pat. No. 4,859,415 granted to Shida et al. It was demonstrated that minor (≤0.15 wt. %) Pd and Ru additions to various high-strength commercial alloys such as Ti-6Al-4V, Ti-6Al-2Sn-4Zr-2Mo, Ti-6Al-6V-2Sn, Ti-6246, and Ti-38644 can measurably elevate threshold temperatures for chloride crevice attack and SCC in deaerated, sour, deep-well brine fluids at higher temperatures. This benefit stems from localized alloy ennoblement and repassivation from these PGMs in hot reducing acid chloride media formed within crevices and cracks to counter the anodic acid-chloride corrosion mechanism.
Unfortunately, this PGM alloy ennoblement effect cannot effectively counter/prevent SCC at lower temperatures (e.g., at room temperature—about 77° F.) in aqueous chloride media, where mixed cathodic/hydrogen embrittlement and/or anodic chloride mechanisms can prevail. In fact, if a titanium alloy has a relatively high aluminum equivalency (i.e., Al+O content) and incurs substantial alpha-two (Ti3Al) compound precipitation, the Ru or Pd alloy additions merely serve to further aggravate chloride SCC and produce low KSCC values. With the exception of the Ti-38644 (beta) alloy listed prior, all of the remaining commercial alpha-beta alloys mentioned can be expected to suffer low fracture toughness (KSCC values) in aerated or deaerated saltwater and brines over a wide temperature range. This negative PGM addition effect can be avoided by adding minor Ru or Pd levels to a lower Aluminum Equivalency (lower Al+O containing) titanium alloy such as Ti-3Al-2.5V (Gr. 9 Ti) or Ti-6Al-4V ELI (Gr. 23 Ti) to produce ASTM Grades 28 and 29 Ti, respectively; which do offer favorable saltwater fracture toughness (i.e., high KSCC values). Unfortunately, reducing the Al+O alloy content sufficiently to minimize or avoid alpha-two precipitation also results in alpha or alpha-beta alloys possessing relatively low strengths (YS≤110 ksi).
As shown in Table 1, although the Ti-6Al-4V-Ru (ASTM Gr. 29) alloy is highly weldable, fracture resistant, and offers exceptional hot brine corrosion resistance to 600° F., the alloy's lower design yield strength (YS) of 110 ksi and significant degradation of YS with increasing temperature (e.g., 78 ksi at 500° F.) translate into a substantial tubular wall thickness increase and weight penalty particularly as HPHT/XHPHT service temperatures exceed ˜300° F. Table 1 shows various higher-strength (more highly alloyed) commercial alpha-beta titanium alloys offering a 130 ksi minimum YS in the fully transformed-beta plus STA condition, and exhibiting limited finite fusion weldability. While Table 1 shows that the Ti-662 alloy has some desirable characteristics, this classic aerospace alloy exhibits very poor/limited resistance to localized corrosion attack and stress corrosion cracking (i.e., low KSCC) in aqueous chloride media, especially as temperature increases. In addition, Ti-662 nominally contains 0.6 wt. % Fe and 0.6 wt. % Cu (for increased aged strength), which can cause substantial elemental micro- and macro-segregation/inhomogeneities during melting of larger ingots needed for energy industry components. As overviewed in Table 1, the inventors are unaware of any prior commercially-available higher strength titanium alloys which meet various criteria desired for successful use in the field of energy extraction.